1. Field of the Invention
The present invention relates to a screwing - unscrewing apparatus, in particular for screwing-unscrewing the studbolts securing the cover of the reactor vessel of a nuclear reactor, of the type comprising a frame, at least one traction assembly, at least one screwing-unscrewing assembly and at least one support assembly for a stud-bolt, which are mounted on the frame, said traction assembly comprising a traction cylinder having a body fixed vertically to the frame and a traction head which is connected to the piston rod of the traction cylinder and which is operative to grip an upper part of a securing bolt for exerting a vertical pull thereon, said screwing-unscrewing assembly comprising a rotational drive mechanism operative to come into drive relation with the upper part of the stud-bolt, and said support assembly being operative to grip the upper part of the stud-bolt for supporting it.
2. Description of the Prior Art
In nuclear reactors, the cover or lid is removably fixed to the reactor vessel so as to allow, from time to time or periodically, inspection of the inside of the vessel and/or of the members situated inside the vessel and for allowing repair thereof if required. This securing is achieved by a large number of stud-bolts, in general 58 stud bolts in a reactor of the P.W.R type.Usually, each studbolt extends vertically and comprises, in its lower part, a cylindrical threaded portion extended by a smooth truncated cone shaped portion for facilitating engagement of the stud-bolt in a tapped hole in the upper edge of the vessel, and, in its upper part, another cylindrical threaded portion on which is screwed a locking nut.
During securing of the lid, each stud-bolt is caused to pass through a smooth hole in the lid, then each studbolt is engaged and screwed in one of the tapped holes in the vessel. Then, a strong pull is exerted on each stud-bolt, then the nut associated with the bolt is turned by a few turns so as to be placed in contact with the lid without clamping. Then, the pull exerted on each stud-bolt is released. For removing the lid, the reverse operations are carried out.
The screwing-unscrewing apparatuses at present in use, which allow the above mentioned operations to be carried out are either automatic or semi-automatic. In the automatic version, a traction assembly, a screwing-unscrewing assembly and a support assembly are associated with each stud-bolt. In the semi-automatic version, a traction assembly is associated with each stud-bolt, and a limited number of screwing-unscrewing assemblies and a corresponding limited number of support assemblies, for example one or two assemblies of each, are provided which are brought selectively and successively into functional relation with the stud-bolts.
In the two automatic and semi-automatic versions, the operations for screwing or unscrewing the stud-bolts are uncertain. In fact, because of the manufacturing tolerances of the lid, of the tapped holes in the vessel and of the screwing-unscrewing apparatus itself, the axes of the active elements of the screwing-unscrewing apparatus never tally perfectly with the axes of the tapped holes in the vessel or with the axes of the stud-bolts. The result is that, despite the presence of a smooth truncated cone shaped part at the lower end of each stud-bolt, it often happens that centering of the stud-bolt can not be effected automatically during fitting of the stud-bolt into the tapped hole of the vessel, and that this operation must be carried out manually as well as the first screwing up turns of the stud-bolt. Similarly, during an operation for unscrewing the stud-bolt, it happens that the active elements of the screwing-unscrewing apparatus cannot correctly grip the upper part of the stud-bolt and must therefore be positioned manually with respect thereto. Furthermore, since the axes of the traction cylinders and of the other active elements of the screwing-unscrewing apparatus do not tally perfectly with the axes of the tapped holes in the vessel, these alignment defects result in the parts driven with a relative movement being subjected to stresses which are unevenly distributed about their translational or rotational axis, thus causing, by friction, uneven and premature wear of these parts.
Another problem resides in the fact that each stud-bolt must be subjected to a strong pull at a given moment during the screwing or unscrewing operation. In the apparatus at present in use, this strong pull is obtained by means of a traction cylinder having a single piston of relatively large diameter. Because of the relatively large number of studbolts and because of their relatively small mutual spacing, the use of large diameter traction cylinders makes it necessary to dispose the traction cylinders in positions alternatively offset in height. Furthermore, since the diameter of the traction cylinders is necessarily limited by the small mutual spacing between the stud-bolts, the traction cylinders must be supplied with fluid under a very high pressure, in general 1,5.10.sup.8 Pa or more, so as to obtain the required pulling force. This increases the cost and the complexity of the hydraulic feed systems in so far as construction, sealing and safety are concerned.
A first object of the present invention is to solve the alignment problems by a relative judicious arrangement of the traction assembly, the screwing-unscrewing assembly and the support assembly of the screwing-unscrewing apparatus.
A second object of the invention is to allow automatic alignment of the different active elements of the screwing-unscrewing apparatus with respect to the axis of the stud-bolt or of the tapped hole in the vessel so as to reduce, even completely eliminate the causes of wear of the parts moving relatively to each other and so as to eliminate any manual positioning or centering operation.
A third object of the present invention is to provide a screwing-unscrewing apparatus in which the traction cylinders have a diameter such that they may be disposed circumferentially side by side, at the same height, and so that they may operate at a substantially lower pressure than in previously known apparatuses,while still being capable of supplying a pulling force as high as in the previously known apparatuses.